Swolf is derived from combining the terms swimming and golf. The swolf data field on the Forerunner 910XT and Garmin Swim adds the time and the number of strokes it takes to swim a pool length. For instance, 30 seconds and 10 strokes to swim the length of a pool will equal a swolf score of 40. A lower score is better, just as in golf.
The Forerunner 910XT can only utilize swolf when set to Pool Swim within the Swimming sport mode.